1. Field of the Invention
The present invention relates to a sealing member for a cap featuring excellent cap performance as well as excellent safety and environmental friendliness, and to a metallic cap.
2. Description of the Related Art
A cap has a sealing member such as a gasket formed in a portion to where at least the mouth of a container comes in contact to maintain sealing performance. Most of the conventional caps are employing a sealing member made of a plastisol of vinyl chloride since it has a suitable degree of elasticity and workability.
In the case of a metallic cap, the inner surface of the cap comes into direct contact with the content and must, hence, protect the metal body and must prevent the metallic component from eluting out. For this purpose, a resin film has been formed on the surface of the metal body. As the film applied onto the inner surfaces of the caps, there has been used an epoxy film or a vinyl chloride resin film owing to their adhesiveness and workability.
However, the vinyl chloride is a substance that could cause the formation of dioxin and is further often treated by using a large amount of a plasticizer such as dioctyl phthalate which is harmful to the human body. Further, the epoxy resin used for the film uses a bisphenol A (BPA) which is becoming a problem as an environmental hormone, a bisphenol glycidyl ether (BADGE) which is harmful to the human body and a novolak glycidyl ether (NOGE) accompanied by such a probability that these substances may elute out. It is desired to avoid their use particularly as the sealing member or the inner film of the caps that are used in the field of foods.
It has also been known to use a polyurethane resin as a sealing member for the caps (Japanese Unexamined Patent Publication (Kokai) No. 9481/1986). This prior art uses a polyurethane elastomer which contains an aromatic isocyanate.
In order to offer satisfactory sealing performance, easy-to-open performance and durability, on the other hand, the sealing member for the caps must have a hardness of from 10 to 70 (JIS A hardness), a tensile strength of from 1 to 40 MPa and a compressive permanent distortion in a range of from 0.1 to 60%.
There, however, exists a correlation between the hardness and the compressive permanent distortion, and it is difficult to set their properties to lie within the above optimum ranges. That is, there is available no sealing member that satisfies all of the sealing performance, easy-to-open performance and durability. If it is attempted to obtain a polyurethane elastomer containing the aromatic isocyanate and having a low hardness, then, the mechanical strength decreases. Besides, the polyurethane elastomer is colored through the aging.